COSMOLOGY
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Q1. What is the paradox of Olbers’ Paradox? How is it resolved? Answer
Q2. Why is it surprising that it gets dark at night? Answer
Cosmology - Big Bang
Theory Questions
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Q1. In general terms, what are the two possible outcomes for the expanding universe? Which outcome is currently favored by observations? Answer
Q2. What observational evidence is there to support the idea that a big bang occurred? Answer
Q3. Describe in very general terms the evolution of the universe, according to the big bang theory. Answer
Cosmology -
History of the Universe Questions
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Q1. The big bang theory predicts that the universe will occasionally undergo abrupt transitions in the state of matter in the universe. Why do these changes occur, in general? What physical change occurred in the last two transitions? What observations tell us that these transitions actually occurred? Answer
Q2. Describe the observation which tells us that the universe underwent a significant transformation a few hundred thousand years after the expansion began. How is this observation predicted by the big Bang theory? Answer
Q3. How did the properties of matter in the universe change during the last (most recent) transition in the universe? How can we tell if this transition actually occurred? Answer
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Q1. Describe the observations which tells us that the universe is expanding. How can we use these observations to determine how long the expansion has been occurring? Answer
Q2. What observations constitute Hubble’s Law? How is it interpreted to describe the universe? Answer
Q3. If a galaxy is observed to have a velocity V1, what would its velocity have been a billion years ago? Explain. Answer
Q4. What is Hubble’s Law? What does it tell us about the nature of the universe? Answer
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Q1. What is a standard candle and how are they used to measure distances? What is meant by the step ladder approach to measuring distance, and what are its consequences in astronomy? Answer
Q2. How are distances measured in astronomy? Why is a knowledge of the distance to a star important? Answer
Distances - Parallax Questions
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Q1. How is the distance of a nearby star determined? Why is it important to be able to measure the distance to stars? Answer
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Q1. What is a Cepheid variable star and how is it used to measure distances to galaxies? Answer
Q2. How were distances within and between galaxies first measured? What is the unique property of the stars involved in this determination? Answer
Q3. What are the properties of Cepheid variable stars? In what way are they important to the study of galaxies? Answer
Cosmology - General Answers
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A1. If the universe is truly infinite in every respect, then every line of sight should end on the surface of a star and the night sky should be everywhere as bright as the surface of the sun. Clearly that is not the case. The only resolution of this paradox is that some property of the universe must be finite.
A2. If the universe were truly infinite in all its aspects, then we should see the surface of a star in every direction we look in the sky. That would make the sky blazingly bright in every direction. This is clearly not the case, which implies that the universe is not infinite in all its aspects.
Cosmology - Big Bang Theory Answers
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A1. The universe may either expand for ever, with matter becoming ever more spread out, or the expansion may one day stop and begin to contract back toward another big bang. It appears that the universe is very close to the transition case, where the universe expands for ever, but at an ever decreasing rate.
A2. The primary evidence to support the big bang theory is the existence of the background radiation and the abundance of deuterium in the universe. The background radiation was released when atoms became electrically neutral almost a million years after the big bang. Its uniformity tells us it comes from a time before irregularities had developed in the matter of the universe. Deuterium (the heavy form of hydrogen) is a by-product of the nuclear reactions that occurred about three minutes after the big bang, when most of the helium of the universe was created. The amount of deuterium and other light elements left over from those reactions depends sensitively on the total density of the universe.
A3. The universe began as a very compact space with all matter and energy contained within it. That space has been expanding at the speed of light for the past 12-15 billion years. The expansion has been steady, and has led to a steadily decreasing average temperature for the universe. As certain critical temperatures have been reached, the properties of the universe have undergone abrupt transitions.
Cosmology - History of the Universe Answers
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A1. As the universe expands, the matter of the universe cools. When certain critical temperatures are reached, the properties of the matter undergo an abrupt change, such as when cooling water turns from liquid to solid at 32 oF. The last two transitions, in reverse order, are the formation of neutral atoms when electrons were captured into orbits around the nuclei and the nuclear reactions that formed elements from hydrogen. When the atoms became neutral they also lost their ability to interact with the light of the universe. As a result the light present at the time of the transition (characteristic of matter at the temperature of the transition) still exists today and is visible as a faint uniform background glow, shifted by the expansion of the universe into the microwave portion of the spectrum. Nuclear reactions converted some of the hydrogen into helium, resulting in the 90/10 mix of hydrogen/helium we see in the universe today. More significantly, a few nuclei would have been "caught in the middle" when the reactions ended. These intermediate products are very scarce, but their presence tells us that the reactions occurred very quickly and stopped suddenly, just as predicted by the big bang theory.
A2. Astronomers observe a (nearly) uniform background radiation characteristic of a black body radiator with a temperature of 3K. This observation is interpreted as the relic of the radiation that existed in the universe at the time that neutral atoms were formed, redshifted into the microwave by the extreme distance it has traveled. When atoms became neutral, the matter of the universe became transparent to most forms of light and the radiation stopped cooling as the universe continued to expand.
A3. The last transition of the properties of the universe occurred when neutral atoms formed. Before the transition, free electrons and bare nuclei were mixed together. As a result of binding the electrons within atoms, the matter of the universe became transparent. The light of the universe from that time still exists today, in the form of a uniform black body radiation characteristic of the temperature when the atoms were formed, but red shifted according to Hubble’s Law into the microwave portion of the spectrum. This background radiation has been observed, with exactly the properties predicted.
Hubble's Law Answers
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A1. The observations contained in Hubble’s Law demonstrate that all galaxies outside our own group of galaxies are moving away from us, with speeds that are larger for greater distances from us. The greater speeds at greater distances are the result of those galaxies having moved further during the time of their existence because of their greater speed. This idea suggests that the age of the expansion can be estimated as the length of time required for a galaxy to move to its present distance at the speed we observe.
A2. Hubble’s Law is based upon observations of the distance and velocity of individual galaxies. These observations show that all galaxies outside our group of galaxies are moving away from us, with greater speeds at greater distances. This observation is interpreted as showing that we live in an expanding universe, with every galactic object moving away from every other object. No galaxy, including our own, occupies a special or unique location in the universe.
A3. The velocity a given galaxy has is the velocity it was given at the time of the big bang. It has been floating ever since. Hence, the velocity we see today for a galaxy is the same as the velocity it would have had a billion years ago. If the universe is especially dense, the gravity of the universe might have slowed the expansion slightly in that period, slightly slowing the galaxy.
A4. Hubble’s Law states that the velocity of a galaxy away from us is proportional to its distance from us. This implies that the universe is expanding, so that all objects are moving away from all other objects in the universe.
Distances Answers
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A1. A standard candle is an object which always has the same actual brightness. Examples include certain types of supernovae explosions, the brightest cluster or nebula in a galaxy, or the brightest galaxy in a cluster of galaxies. Once such an object is identified, its true brightness is automatically known from its membership in the class. A comparison of its actual brightness and its apparent brightness allows the distance to be easily determined.
Each new method for determining greater distances in astronomy must be calibrated by previous distances. For example, the distance to at least one Cepheid variable star must be known before the scale of the relationship between period and actual brightness is determined. Thus, any errors in previous methods are carried over to and accumulated in the next method. For great distances, the accumulated errors can be quite large.
A2. The distance of a nearby star is determined from its parallax. By observing the star from opposite sides of Earth’s orbit around the sun, we can measure the change in its apparent position or parallax. If we know the size of Earth’s orbit, we can use geometry to find the distance to the star. The distance of the star determines its apparent brightness in our sky, compared to its true brightness. We must know its true brightness (how much total light it emits) in order to understand its properties.
Distances - Parallax Answers
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A1. The distance of a star is determined by measuring its parallax. The parallax is the apparent change in the position of a star when seen from opposite sides of Earth’s orbit around the sun. Only when we know the distance to a star can we determine its true brightness from its apparent brightness. If we do not know the distance to stars, we might mistake a nearby but dim star for a more distance but brighter star.
Distances - Cepheid Answers
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A1. A Cepheid variable star changes its brightness in a regular, repeating pattern because the star is pulsating, changing in physical size and brightness. The length of time required for one cycle of change is directly related to the average actual brightness of the star. Thus, if the period is determined from observations, the actual brightness can be easily found. The difference between actual brightness and the apparent brightness of the star allows the distance to be easily determined.
A2. Distances within and between galaxies are measured using Cepheid variable stars. These stars change in brightness with a regular period, and the length of this period is directly related to their average true brightness. Hence, their distance can be determined from the difference between the true brightness determined from the period and their apparent brightness.
A3. A Cepheid variable star pulsates in size, following a regular pattern. As it changes size, its brightness also changes in a regular pattern. The length of time a given Cepheid variable takes to go through one cycle of pulsation is directly related to its brightness. This makes them a standard candle, which can be used for measuring distances. Since some Cepheid variables are very bright, they can be found in any galaxy close enough to observe individual stars. Once their period is determined, we can use them to measure the distance to the galaxy.